The methionine (MET) cycle is central to methylation in developmental processes (e.g. gene-environment interactions). Impaired methyl transfers have structural and metabolic consequences e.g. neural tube defects and high homocysteine (HCYS). Gene methylation is a fundamental mechanism leading to persistent epigenetic changes. Maternal under-nutrition and IUGR results in epigenetic changes in fetal tissues, including decreased methylation that increases gene expression. We have developed a baboon non-human primate (NHP) model of moderate maternal nutrient restriction (MNR) resulting in IUGR. MNR mothers eat 70% of controls (CTR) eating ad lib. In MNR, fetal vitamin B12 is decreased 22% while folate is unchanged. Rat studies show hyperhomocysteinemia is associated with increases in both plasma HCYS inflow and outflow. However NHP data are needed since rat 1-CC function differs from primates. Central to our proposal to develop these techniques in an IUGR NHP model, rat studies in pregnancy show that plasma fluxes under- estimate by >80% absolute tissue HCYS production e.g. little maternal or fetal hepatic HCYS is exported to plasma. Thus, plasma HCYS does not reflect major MET cycle tissue activity. As a consequence, MET cycle dysfunction does not necessarily lead to elevated HCYS and can occur with normal plasma HCYS. B12 is essential for methyl group transfer from 5-methyl-tetrahydrofolate to methylate HCYS to MET. HYPOTHESIS: MNR decreases B12 availability to the fetal MET cycle decreasing 1) overall HCYS methylation 2) contribution of HCYS methylation in maternal and fetal tissues to plasma HCYS. APPROACH: In stable isotope studies at 0.9 gestation in pregnant baboons chronically instrumented with maternal and fetal catheters maintained on a tether system, we use a priming dose followed by constant infusion of the tracers [U-13C] MET and [1-13C] HCYS to determine HCYS methylation and HCYS kinetics in both CTR and MNR pregnancies. Our collaborators methods in pregnant rats are now in Press. INNOVATION: Human studies cannot 1) control and match pre-study maternal phenotype, 2) precisely control nutrient intake; 3) obtain maternal and fetal tissues. Novelty lies in combining a unique NHP model and a pressing developmental question. R03 RESPONSIVENESS: The R03 mechanism encourages novel, high-risk studies. These isotope techniques have never been performed in any precocial species. INVESTIGATORS have many years experience with perinatal baboon studies and have published extensively on this NHP MNR/IUGR fetal phenotype. ENVIRONMENT: Our baboon feeding facility is unique worldwide to produce controlled MNR and IUGR. RELEVANCE: Primate and rodent MET cycle metabolism differs markedly. Identification of alterations in tissue methylation will provide evidence of tissues in which MET cycle dysfunction occurs with resultant epigenetic changes. Information will translate to human pregnancy aiding diagnostic, preventative and therapeutic strategies. B12 has been ignored in comparison to folic acid.